To understand language, children must learn to listen for meaning in sequences of speech sounds that unfold rapidly in time. Fluent understanding requires "listening ahead", continually anticipating what is coming next by using linguistic and nonlinguistic information from the context in which words are spoken. Skilled adults draw on multiple sources of knowledge in order to process speech with such remarkable speed and efficiency, but little is known about the early development of this critical capacity. The first goal of the research is to investigate the development of spoken language understanding by English-learning children from 18-30 months, using powerful new methods for measuring the time course of word recognition by monitoring children's eye movements as they listen to speech. Efficiency in comprehension increases as the child learns to take advantage of regularities in the structure of continuous speech, enabling the child to anticipate upcoming words and relate word meanings to one another. The three general hypotheses motivating the proposed experiments are: 1) that infant-directed (ID) speech provides the inexperienced listener with perceptual regularities useful in early speech processing; 2) that children use their emerging knowledge of linguistic regularities in the language they are learning to facilitate word recognition; 3) that the rapid processing of predictable elements in the speech stream increases children's capacity to attend to and learn about novel elements. The first l0 experiments ask whether early word learning is enhanced by common features of ID speech such as exaggerated intonation and one-word utterances, and how children make use of language-specific cues in English such as sentence stress, morphosyntactic regularities, word order, and semantic information in understanding fluent speech. The second goal of the project is to integrate laboratory studies of infant speech comprehension with cross-language field research on ID speech, extending this research to Spanish. An observational _study of Hispanic mothers' speech to infants will inform the hypotheses to be tested in 7 experiments on speech processing by Spanish-learning children from 18 to 30 months. These experiments explore how infant word recognition is influenced by characteristic features of Spanish ID speech, and how Spanish-learning children make use of language-specific cues such as gender- and number-marking, word order, and semantic information in understanding fluent speech. The third goal is to refine newly developed experimental procedures for monitoring on-line speech comprehension by young language learners, and to evaluate their potential clinical relevance for assessing disorders in early language development.